Transdermal delivery of a therapeutic agent such as a drug through the skin to the local tissue or systemic circulatory system without piercing the skin, such as with a transdermal patch, has been used successfully with a limited number of therapeutic molecules. The main barrier to transport of molecules through the skin is the stratum corneum (the outermost layer of the skin).
Devices including arrays of relatively small structures, sometimes referred to as microneedles or micro-pins, have been disclosed for use in connection with the delivery of therapeutic agents and other substances through the skin and other surfaces. The devices are typically pressed against the skin in an effort to pierce the stratum corneum such that the therapeutic agents and other substances can pass through that layer and into the tissues below.
Microneedle devices having a fluid reservoir and conduits through which a therapeutic substance may be delivered to the skin have been proposed, but there remain a number of difficulties with such systems, such as expense and the ability to make very fine channels that can reliably be used for fluid flow.
Microneedle devices having a dried coating on the surface of a microneedle array have also been developed. The devices are generally simpler than microneedle devices having fluid reservoirs and may directly introduce a therapeutic substance into the skin without the need for providing reliable control of fluid flow through very fine channels in the microneedle device.